Wow, I go on vacation for a few days and the whole design has changed
I like the direction this has gone,

Seems we have deviated into 2 seperate designs, one for a pistol and another for a "machine gun"

For the pistol, I also don't think the arbor pin has the diameter to include a durable piston/spring arrangement as is. But there is nothing saying the design couldn't have a smaller caliber and larger arbor pin in roughly the same sized cylinder. With the gas-tube mounted under the barrel, it may start to look somewhat like a LeMat revolver.
As for delaying the action. I think that Stepshanie's suggestion is the simplest way to go about a remedy.
Another solution, if the over-the-top "AK" system is used. couldn't you just add a locking link (ala, 1911) or camming lug (similar to HKs system) to delay the piston?

Maybe i missed a few points, as I have fallen a bit behind in this thread